1. Field of the Invention
The present invention relates to an improved process for the preparation of phosphorodihalodithioates that can be used as intermediates for the synthesis of insecticidally active compounds.
2. Brief Description of the Prior Art
Disclosed by the prior art is a process for preparing phosphorodihalothioates by heating the corresponding phosphoric acid alkyl ester dichlorides with phosphorus(V) sulfide to 140.degree.-150.degree. C. (see Houben-Weyl: "Die Methoden der Organischen Chemie" (The Methods of Organic Chemistry), Volume 12/2, page 682 [1964], George Thieme Verlag Stuttgart). Alternately, O-alkyl ester dichlorides can be reacted directly with phosphorus(V) sulphide to give the dithiophosphoric acid alkyl ester dichlorides.
In carrying out these processes industrially, one finds problems in separating and removing the phosphorus pentoxide by-product. After separating the dithiophosphoric acid alkyl ester dichlorides, there remain solid phosphorus pentoxide and sulphur-containing, extremely malodorous compounds. Their removal to leave an odor-free product, say by oxidation in an alkaline medium, is only partially feasible, and requires long times and high costs.
Another method of preparing dithiophosphoric acid ester dichlorides comprises reacting elemental sulphur with thiophosphorous acid ester dichlorides that are obtained from the reaction of thiols and phosphorus trichloride. The reaction of sulphur with the thiophosphorous acid ester dichlorides only takes place at temperatures above 100.degree. C. The resulting sulphurization of the thiophosphorous ester dichlorides is attended by a marked disproportionation to dithiophosphorous acid diester chlorides and phosphorus trichloride. To suppress the disproportionation, the sulphurization must be carried out under pressure (see Houben-Weyl, loc. cit.).
U.S. Pat. No. 3,879,500 and Russian Patent No. 187,785 disclose what appears to be a simple method of preparing dithiophosphoric acid ester dichlorides. The method comprises reacting corresponding thiol compounds with thiophosphoryl chloride. However, if too large an amount of thiol compound is employed in this reaction, trithiophosphoric acid diester chlorides and tetrathiophosphoric acid esters are obtained, almost exclusively as the reaction product even in the presence of acid-binding agents (see also Houben-Weyl, loc. cit.).
U.S. Pat. No. 4,082,822 discloses a process for the preparation of a dithiophosphoric acid dihalide by reacting a thiol compound with a thiophosphoryl halide in the presence of a catalyst. Distinctly, the catalyst is selected from the group consisting of a metal, an anhydrous metal halide, a Lewis acid, a nitrogen-alkylated lactam or an N,N-disubstituted carboxylic acid amide or phosphoric acid amide at a temperature of about 0.degree. to 170.degree. C.